Date of Award

Winter 2025

Project Type

Dissertation

Program or Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

First Advisor

Gregory P Chini

Second Advisor

Gregory P Chini

Third Advisor

Guillaume Michel

Abstract

Standing acoustic waves in density-stratified fluids confined within horizontal channels generate fluctuating vorticity baroclinically, driving mean streaming flows that are much stronger and qualitatively distinct from classical Rayleigh streaming. Theoretical and numerical studies have shown that these baroclinic acoustic streaming flows can reach speeds comparable to the maximum speed of the oscillatory sound-wave field, enabling fully two-way wave/mean-flow coupling: the streaming flow modifies the background density field, which then alters the structure of the acoustic waves. Extending this framework, Massih (2022) quantified the heat-transfer enhancement arising from baroclinic streaming as a function of the channel aspect ratio (δ). The present work advances this line of inquiry by examining the structure of the wave/mean-flow coupling and the associated heat transport in density-stratified channels of larger aspect ratio (δ ≤ 10) subjected to wall-parallel (“horizontal”) acoustic-wave forcing. The dependence of the heat transport on the amplitude of the acoustic forcing is characterized, revealing regimes of strong two-way coupling. Asymptotic scaling arguments are used to identify parametric dependencies, while detailed analysis is undertaken to elucidate the role of baroclinicity in sustaining the streaming flow. A systematic asymptotic analysis of an acoustically driven, weakly inhomogeneous ideal gas confined in a horizontal channel is then performed to capture the transition from the classical Rayleigh streaming regime to the baroclinic streaming regime, thereby bridging an outstanding theoretical gap in the literature. A closed-form analytical solution is derived and validated against prior experimental and numerical results, revealing the distinct contributions of viscous and baroclinic torques and yielding the critical temperature-gradient scaling governing the transition. Finally, the wave-averaged linear stability of density stratified gases subjected to wall-normal (“vertical”) acoustic forcing is investigated. A fully two-way coupled stability formulation is developed to account self-consistently for feedback between acoustic and thermal perturbations. The analysis accurately predicts the onset and spatial structure of baroclinically driven instabilities, in excellent agreement with nonlinear simulations.

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